There are 6 rub pads on the backing plate (3 for each shoe,top middle and bottom). The sides of the shoes rub on the pad area. A real light film of syl-glyde or antisieze will help if it is that making the sqeak noise.
The other thing it could be is dust from the new shoes seating in is floating around between the shoe and drum surface. It may need to be cleaned out and one more thing,,,
Have you adjusted your rear shoes recently?
Some are auto adjusted with the use of the park-brake
